1. Field of the Invention
The present invention relates to a rubber blanket for an offset printing.
2. Description of the Related Art
A rubber blanket for an offset printing plays the role of receiving an ink from a printing plate and transferring the received ink onto a paper sheet or the like. A rubber blanket constructed as shown in, for example, FIG. 4 is known to the art. As shown in the drawing, the rubber blanket for an offset printing comprises a surface rubber layer 11 for receiving and transferring an ink. Laminated on the lower surface of the surface rubber layer 11 are a solid rubber layer 141, a fabric layer 121, a solid rubber layer 142, a compressible layer 13, a solid rubber layer 143, a fabric layer 122, a solid rubber layer 144, a fabric layer 123, a solid rubber layer 145, and a fabric layer 124 in the order mentioned.
The compressible layer 13, which permits improving the absorption of the shock and reproducibility of the printing plate, is formed of, for example, a porous rubber. Each of the four fabric layers 121 to 124 is formed of, for example, a natural fiber such as cotton or a woven fabric such as a synthetic fiber. On the other hand, each of the solid rubber layers 141 and 142 performs the function of an adhesive agent for bonding the adjacent fabric layers to each other.
It is necessary for the rubber blanket for printing, which is mounted to a blanket cylinder made of a metal with a predetermined tension, to have a suitable elongation rate and a sufficient mechanical strength. Therefore, it is considered desirable for the rubber blanket for printing to include a large number of fabric layers.
Disclosed in U.S. Pat. No. 5,364,683 is a blanket for printing comprising a first compressible fabric layer prepared by coating the upper surface of a woven fabric with a binder material containing a plurality of cells so as to impart a compressibility to said woven fabric, an intermediate compressible layer laminated on the first compressible fabric layer, a first adhesive layer laminated on the intermediate compressible layer, a second compressible fabric layer laminated on the first adhesive layer and prepared by allowing a woven fabric to be impregnated a binder material containing a plurality of cells, a second adhesive layer laminated on the second compressible fabric layer, and a third compressible fabric layer laminated on the second adhesive layer and prepared by allowing the lower surface region of a woven fabric to be impregnated with a binder material containing a plurality of cells.
However, each of the rubber blanket for an offset printing, which is constructed as shown in FIG. 4, and the blanket for printing disclosed in U.S. Pat. No. 5,364,683 gives rise to problems (1) to (3) given below:                (1) The fabric layer is formed mainly of a woven fabric and, thus, tends to tinked down or packed down so as to be permanently deformed, if pressure is applied thereto. Therefore, if the blanket includes many fabric layers, e.g., 3 to 4 fabric layers, generated is the problem called tinking down or packing down, i.e., the problem that the thickness of the entire blanket is decreased by the pressure repeatedly applied to the blanket during the printing operation. The problem of tinking down or packing down tends to be generated particularly in the case of using a cotton cloth for forming the fabric layer. If the thickness of the blanket is decreased, the smash thickness is lowered so as to lower the degree of the ink transfer, with the result that generated is a defective printing such as failure to obtain a required ink concentration. In such a case, required in many cases is a so-called “cylinder arrangement”, i.e., the operation to dispose an underlay below the blanket so as to restore the thickness of the blanket and, thus, to acquire the normal smash thickness, with the result that the rate of operation of the printing apparatus is lowered.        (2) If printing is performed on a large number of paper sheets having a small width by using a blanket including many fabric layers, i.e., 3 to 4 fabric layers, tinking down or packing down (permanent deformation) is generated in that portion of the blanket which corresponds to the printing paper sheets having a small width. If printing is performed subsequently on a paper sheet having a large width, the printing concentration is lowered in the tinking down or packing down portion of the blanket. In an extreme case, a clear difference in the printing concentration is brought about between the portion of the printed paper sheet corresponding to the portion of the blanket where the tinking down or packing down is generated and the portion of the printed paper sheet corresponding to the portion of the blanket where the tinking down and packing down is not generated, making it unavoidable to discard the printed paper sheet as a defective printed material.        (3) If a paper sheet is broken during the printing operation so as to cause many pieces of the broken paper sheet to be wound around the cylinder of the blanket, the blanket partially receives an excessively high pressure so as to bear a recess. In the worst case, the blanket is bruised. As a result, the recessed or bruised portion of the blanket is rendered substantially incapable of receiving the ink from the printing plate. In this case, the printing cannot be performed so as to make it necessary to replace the blanket. Also, if the defective blanket noted above is applied to an old machine or a high speed machine, an axial streak called shock-streak tends to be reproduced on the printed matter.        